Electrophysiological studies of isolated epithelia are being done to characterize their active and passive pathways for ion transport. In particular, the current-voltage relationships of the frog skin, rabbit cortical collecting tubule and the turtle urinary bladder are being assessed with the working hypothesis that the distinct "breaks" observed at unique voltages in the I-V plane may define the driving forces for active ion transport in parallel transport pathways. Studies are being done to compare the values of the parameters for transport that can be calculated directly from simple equivalent circuits derived according to their I-V relationships with the values of the transport parameters obtained in more conventional ways. From this it should be possible to obtain a better understanding of the interrelationships between ion transporting systems especially in heterogeneous epithelium where several ion species are transported simultaneously and accordingly, a better understanding of the mechanism(s) whereby drugs and hormones alter the capacities for transport. BIBLIOGRAPHIC REFERENCES: Helman, S.I., R.G. O'Neil, R.S. Fisher. Determination of the ENa of frog skin from studies of its current-voltage relationship. (In press, Amer. J. Physiol., Oct. 1975.)